Endothermic and Exothermic Reactions

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Endothermic Reaction Noun

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[en-doh-thur-mik ree-ak-shun]

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Endothermic Reaction

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A chemical reaction that absorbs energy from its surroundings, usually as heat, causing the temperature of the surroundings to decrease.

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Example: This diagram shows an instant cold pack's inner components. Breaking the inner water bag allows it to mix with ammonium nitrate, causing an endothermic reaction that absorbs heat.

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Exothermic Reaction Noun

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[ek-soh-thur-mik ree-ak-shun]

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Exothermic Reaction

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A chemical reaction that releases energy into its surroundings, often as heat or light, causing the temperature to increase.

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Example: The arrows pointing away from the test tube show that energy is being released, which is what happens in an exothermic reaction.

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Reactants Noun

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[ree-ak-tants]

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Reactants

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The initial substances that are consumed during a chemical reaction to be transformed into products.

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Example: This diagram shows a chemical reaction. The substances on the left of the arrow, methane (CH₄) and oxygen (O₂), are the reactants.

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Products Noun

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[prod-ukts]

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Products

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The new substances that are formed as the result of a chemical reaction between reactants.

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Example: This diagram shows a chemical reaction where methane and oxygen (reactants) combine to form new substances, carbon dioxide and water, which are the products.

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Activation Energy Noun

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[ak-tuh-vey-shun en-er-jee]

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Activation Energy

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The minimum amount of energy that must be provided to compounds to result in a chemical reaction.

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Example: This graph shows that activation energy is the 'hill' of energy a reaction must overcome. An enzyme lowers this hill, making the reaction happen more easily.

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Law of Conservation of Energy Noun

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[law ov kon-ser-vey-shun ov en-er-jee]

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Law of Conservation of Energy

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The fundamental principle stating that energy cannot be created or destroyed, only changed from one form to another.

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Example: This image shows two examples of energy transformation: chemical energy in fuel becomes mechanical energy in a car, and an apple's potential energy becomes kinetic energy as it falls.

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Combustion Noun

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[kuhm-buhs-chun]

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Combustion

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A high-temperature exothermic reaction, also known as burning, that releases a significant amount of energy as heat and light.

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Example: A hydrocarbon fuel (wood) reacts with oxygen to produce carbon dioxide, water, and energy (heat and light), which is an exothermic combustion reaction.

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Photosynthesis Noun

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[foh-toh-sin-thuh-sis]

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Photosynthesis

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A biological process in plants that converts light energy into chemical energy, which is stored in glucose molecules.

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Example: This diagram shows photosynthesis as an endothermic reaction by illustrating that sunlight (energy) is absorbed to convert water and carbon dioxide into glucose.

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Thermal Decomposition Noun

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[thur-muhl dee-kom-puh-zish-un]

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Thermal Decomposition

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A chemical reaction where a single compound breaks down into two or more simpler substances when heated.

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Example: Heating a metal carbonate with a Bunsen burner breaks it down into simpler substances. This reaction requires a constant supply of heat, making it an endothermic process.

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Neutralization Noun

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[noo-truh-luh-zey-shun]

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Neutralization

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A chemical reaction in which an acid and a base react quantitatively with each other to form a salt and water.

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Example: This diagram shows an acid and a base reacting to form water and a salt, which is the definition of a neutralization reaction.